It is well known to use asphalt compositions to manufacture waterproofing materials, such as roofing membranes, shingles or underlayments. The purpose of such materials is to offer protection against the elements. Because of its good weatherability and hydrophobic nature, asphalt has been used in waterproofing applications for several centuries.
Roofing materials must possess key mechanical and chemical characteristics, such as pliability, resistance to cracking at low temperatures, high puncture resistance, good strength and elongation (to withstand stresses and potential building movements), resistance to flow at elevated temperatures, and thermal and dimensional stability, etc. Most importantly, a roofing material must have minimal susceptibility to the effects of temperature. It is also desirable that such properties be maintained even after exposure to natural elements, e.g. sunlight, condensation, airborne contaminants, etc., and foot traffic.
It is well known that asphalt by itself does not possess all these essential properties. For example, asphalt undergoes stiffening at low temperatures, which makes it fragile in cold weather temperatures even to light impacts. At high temperatures, instead, the viscosity of asphalt is so low that it flows spontaneously or becomes permanently deformed under minimal mechanical stress. To overcome these problems, modifiers have been added to the asphalt in order to obtain an asphalt composition having more suitable physical and mechanical properties. Polymers commonly used to modify asphalt include amorphous or atactic polypropylene (APP), amorphous polyalphaolefin (APAO), thermoplastic polyolefin (TPO), styrene-butadiene-styrene (SBS), styrene-ethylene-butadiene-styrene (SEBS), synthetic rubber or other asphaltic modifiers that enhance the properties of asphalt. The incorporation of such modifiers into the asphalt widens its workable temperature range for roofing applications and results in improved mechanical and viscoelastic properties. Some of the main reasons for modifying asphalt with polymers are to obtain greater resistance to flow at high temperatures, improved resistance to cracking at low temperatures, high dimensional stability over a wide temperature range, improved durability, and resistance to weathering caused by exposure to the elements.
In the middle of the 20th century, an innovative roofing system utilizing modified asphalt was introduced in the market. This type of roofing system is generally referred to as “modified bituminous roofing,” or “modified asphalt roofing membrane” and eliminates the need for several layers of roofing sheets to be installed on the rooftop. Such roofing materials are comprised of a core that is saturated and coated with modified asphalt. The core is typically a reinforcing carrier made of fabric such as polyester, fiberglass, or a combination of both. The asphalt modifiers typically include atactic polypropylene (APP), amorphous polyalphaolefin (APAO), thermoplastic polyolefin (TPO), styrene-butadiene-styrene (SBS), styrene-ethylene-butadiene-styrene (SEBS), synthetic rubber or other asphaltic modifiers that enhance the properties of asphalt.
The modified bituminous roofing materials described above are used in commercial, industrial and residential applications. Two major classifications of such materials, that are used mostly in industrial and commercial applications are (1) cap sheet and (2) base sheet. The cap sheet takes its name from the fact that its top surface is exposed to the elements. Because their top surface is also exposed to the elements, shingle roofing materials, which are used primarily in residential applications, can also be considered “cap” materials. Both cap sheets and shingles can be manufactured using asphalt modified with APP, APAO, TPO, SBS, SEBS, synthetic rubber or other asphaltic modifiers, and are generally reinforced with a polyester carrier or a combination of polyester and fiberglass. They can be smooth or granular surfaced and are typically greater than 2.8 mm in thickness. Base sheets are typically modified using any of the same modifiers as a cap sheet. Because the base sheet is not intended to be exposed to the elements, the asphalt component is typically modified using less expensive polymers such as APP, or with smaller quantities of polymers such as SBS. The base sheet is typically reinforced with a fiberglass carrier, which is significantly less expensive than polyester. It is smooth surfaced and typically between 1.0 mm to 2.5 mm thick depending upon the job specifications.
In a typical field installation, a base sheet is first applied to the roof deck using mechanical fasteners, hot mopping or cold application techniques. Cap sheets or shingles are applied on top of the base sheets, with the seams of adjacent rolls in offset relation. Underlayments, which are typically reinforced with fiberglass, but can also have no carrier are more commonly used under shingles. Most APP modified asphaltic membranes are torch applied by heating the back side of the sheet to melt the compound and using the molten compound to form a heat weld. Most SBS modified asphaltic membranes are set by hot mopping, torch-application or adhesion using cold-process adhesives as described in U.S. Pat. No. 5,807,911, to Wentz, et al.
SBS is one of the most commonly used asphalt modifiers. It is a made-on-purpose polymer that is widely used in several applications such as in the manufacture of shoes. SBS-modified asphaltic blends have improved low temperature flexibility, pliability and elongation properties. However, blends containing SBS as the only modifier also have disadvantages. For example, they tend to be soft and thus difficult to install during warm weather. They have poor resistance to ultraviolet light from the sun, and thus need to be protected with a coating or mineral granules on the upper surface. Furthermore, SBS's unique molecular structure makes processing SBS polymer difficult, and consequently manufacturing an SBS-asphalt compound requires special mixing equipment.
APP is another commonly used modifier in asphalt blends. It is a by-product in the production of isotactic or crystalline polypropylene (IPP). Because of its low cost and excellent characteristics in modifying asphalt, it has been used for asphalt modification for over forty years. APP modified asphalt blends have higher resistance at elevated temperatures than SBS modified asphalt blends, improved low temperature properties, good heat aging and long-term weatherability characteristics, and excellent walkability on warm days. APP modified asphalt compounds also possess better ultraviolet resistance than their SBS counterparts. Asphalt modified with only APP, however, does not have enough hardness, and is susceptible to deformation under minimal mechanical force, especially at high temperatures. Thus when modifying asphalt, APP modifiers are usually used in conjunction with other ingredients such as ethylene-propylene copolymer and IPP. Addition of IPP to an APP-asphalt blend imparts hardness and rigidity to the blend. Incorporation of ethylene-propylene copolymer improves its low temperature flexibility. Depending upon the type of asphalt and the desired final properties, varying proportions of the different modifiers are added to asphalt.
Of the two general types of asphaltic sheet materials used for roofing applications, i.e., APP modified or SBS modified, the SBS based products are more elastic, and have greater flexibility at low temperatures. APP based products, on the other hand, are more resistant to heat (due to a higher softening point), atmospheric effects (especially ultra-violet rays), and foot traffic.
The cost of polymeric modifiers is the highest of all ingredients in the asphalt formulation and therefore has a significant effect on the total cost of the formulation. In order to reduce costs, attempts have been made to include cheaper materials as modifiers, for example, polyethylene. Attempts to use polyethylene as a modifier, however, have been limited to using low density polyethylene as the primary modifier or using it in conjunction with other modifiers such as APP, IPP and ethylene propylene copolymer in the formulation. The primary purpose of such attempts was to replace more expensive and scarcer raw materials such as APP. While APP was originally the unwanted by-product of crystalline polypropylene synthesis, it is becoming increasingly scarce because processes for making IPP have been improved to the point where production of by-product APP is negligible. Despite these attempts and the need to find lower cost materials, to the best of the applicants' knowledge, no modified asphalt roofing membranes composed of asphalt modified with polyethylene as the primary modifier have yet been marketed. There is therefore a need for a polyethylene-based modified asphalt composition which has properties that are ideally suited for modified asphalt roofing systems.